There has been provided a tandem type color printer having four photosensitive drums integrally detachable from a main casing and corresponding to each of four colors yellow, magenta, cyan, and black.
In this type of color printer, a frame is provided in the main casing such that the frame can be pulled out from the main casing in a horizontal direction, for example. The four photosensitive drums are supported on the frame and juxtaposed with one another in a pull-out direction of the frame. Developing cartridges corresponding to the photosensitive drums are detachably mounted onto the frame from above. Each developing cartridge has a developing roller and accommodates toner therein. When any of the developing cartridges runs out of toner, then the frame is pulled outwardly from the main casing, and the developing cartridge is detached from the frame, and a new developing cartridge is mounted onto the frame.